1. Field of the Invention
The present invention relates to a method of controlling a servomotor for driving a feed drive mechanism included in a machine tool or a robot arm of an industrial robot. More specifically, the present invention relates to techniques for improving a delay in a response to a command signal for reversion of the servomotor.
2. Description of the Related Art
A control system for a servomotor included in a machine tool provides with a position control loop which makes the deviation of a position feedback signal provided by a position transducer from an reference position reduced to zero. A velocity control loop provided a velocity feedback signal produced by differentiating the position feedback signal sends an output signal of a velocity loop gain as a torque signal to the servomotor.
It is unable to reverse a moving direction of a movable body which driven by a servomotor for a circular interpolation feed motion, when the rotating direction of the servomotor is changed. The delay is due to lost motions and friction in the feed drive mechanism. When the movable body moves along a circular path from one quadrant to an adjacent quadrant (such movement of the movable body will be referred to as movement for quadrant change) during machining, an actual path of the movable body deviates and bulges out from a desired path specified by a path command as shown in FIG. 7. This phenomenon is called a stick motion or a quadrant protrusion and is one of factors of the deterioration of accuracy in contour machining. Such a phenomenon is presumed to be attributable to the temporary stoppage of the feed drive mechanism due to delay caused by the response characteristic of the velocity loop.
As shown in FIG. 8, a conventional correction method, which has been employed in reducing the quadrant protrusion, detects a torque signal AT at the moment when the servomotor is reversed as an inversion torque signal. The reference torque for correction is obtained by multiplying the torque signal AT by a predetermined correction coefficient.
However, the inversion torque signal at the moment of the reversion cannot be accurately detected when the servomotor is operated for a rapid feed operation, which is affected greatly by inertial force, or when the machine tool has a vertical axis on which exerts the effect of gravity. Especially, the inversion torque signal at the moment of reversion cannot be detected and correction cannot be achieved if the torque signal has a negative value due to the influence of the inertial force.
It was found through experiments on a torque correction at the reversion of the servomotor under conditions of different feed rates and circular arcs of different radii that an inward protrusion occurs before the quadrant protrusion under some feed rate and a circular arc of some radius.